1. Field of the Invention
The present invention relates to an analog-to-digital converter, and more specifically to an analog-to-digital converter for converting an input analog signal into a digital signal of a small bit number at a sampling rate which is considerably faster than the band of the input analog signal.
2. Description of Related Art
Conventionally, it has been an ordinary practice to convert an analog signal into a digital signal by sampling the input analog signal at a sampling frequency which is a double or triple of a maximum frequency of the input analog signal so that the digital signal obtained will have a high resolution. In this analog-to-digital conversion, since signal components of a frequency not smaller than one half of the sampling frequency are included as noise in the digital signal obtained, the input analog signal is previously filtered by a high precision filter so that the signal components of a frequency not smaller than one half of the sampling frequency are sufficiently attenuated before the analog-to-digital conversion. However, it is very difficult to form the high precision filter in an integrated circuit.
Under this circumstance, it has lately been proposed to convert an input analog signal into a digital signal by use of a relatively simple analog-to-digital converter at a frequency sufficiently higher than that of the analog signal and then to pick up only signals within a frequency band of the analog signal by means of a digital filter. The digital filter can have a high precision and reproductivity if the clock frequency is stable.
As the analog-to-digital converters of the type mentioned above, there have been known a variety of types such as a delta-sigma modulator type, a delta modulator type, a first-order predicate primary noise shaping type, etc.
A typical delta-sigma modulator type analog-to-digital converter includes an adder having an input connected to an analog signal input terminal and one integrator having an input connected to an output of the adder. An output of the integrator is connected to a comparator or threshold detector which operates to convert a received analog signal into a digital signal at a given sampling frequency. The digital signal is applied to a flipflop so that the digital signal is held for one period. An output of the flipflop is inverted by an inverter and then supplied to a second input of the above mentioned adder so that a voltage corresponding to a difference between the analog signal of the input terminal and the one-period delayed output of the comparator is integrated by the integrator.
In the above mentioned analog-to-digital converter, if the sampling frequency is very high, and assuming that the period of sampling is T and that the angular frequency of the input signal is .omega., a spectral power density of the noise is in proportion to sin.sup.2 (.omega.T/2) and distributed in a biased frequency range higher than the band of the signal. Therefore, the degree of resolution is lower than that of the conventional analog-to-digital converter, but if it is evaluated in a band of the signal finally obtained, the in-band noise is low and a signal-to-noise ratio is high.
The first-order predicate primary noise shaping type analog-to-digital converter was proposed by Yukawa, A. et al in the International Conference on Acoustics, Speech, and Signal Processing in 1985. This first-order predicate primary noise shaping type analog-to-digital converter and the delta-modulator type analog-to-digital converter are advantageous, similarly to delta-sigma modulator type analog-to-digital converter, in that the sampling frequency is very high and a spectral power density of the noise is distributed in a biased frequency range higher than the band of the signal. Therefore, even if the degree of resolution is lower than that of the conventional analog-to-digital converter, if it is evaluated in a band of the signal finally obtained, the in-band noise is low and a signal-to-noise ratio is high.
However, the above mentioned three types of analog-to-digital converters have one common problem: When the input signal is small, a quantization noise is much distributed within the signal band because of the low resolution of the analog-to-digital conversion. As a result, the signal-to-noise ratio and the input/output amplitude characteristics are inevitably deteriorated.
In order to overcome this problem, it has been known to add a constant DC bias to the input, or to add a sine wave or rectangular wave signal having a frequency which is 1/2.sup.p (p=integer) of the sampling rate and exceeds the band of the signal. This method is effective to some extent in preventing the deterioration of the above mentioned characteristics. In the latter method, however, the signal-to-noise ratio is remarkably decreased in a certain amplitude of the input signal which is determined by the amplitude and the frequency of the additional signal. As a consequence, the analog-to-digital converter as mentioned above cannot fulfil the required performance.
As mentioned above, the conventional analog-to-digital converters are disadvantageous in that an excellent signal-to-noise ratio and input/output amplitude characteristics cannot be realized for a small signal.